Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), flash memory, and resistance variable memory, among others.
Resistance variable memory devices may be used as non-volatile memory for a wide range of electronic applications. Resistance variable memory devices can include phase change random access memory (PCRAM) and resistive random access memory (RRAM), among others. A physical layout of a PCRAM device may resemble that of a DRAM device where the capacitor of the DRAM cell is replaced by a phase change material, e.g., Germanium-Antimony-Telluride (GST) or other chalcogenide materials. That is, an access device such as a diode or metal oxide semiconductor field effect transistor (MOSFET) can be connected in series with the phase change material. Chalcogenide materials can include compounds of sulfides, selenides, and tellurides, among others. GST has been used in rewriteable optical discs, e.g., rewritable compact discs (CD-RW) and rewritable digital versatile discs (DVD-RW), among others. Development is underway for the use of GST in memory cells for RAM applications. A physical layout of an RRAM device may include memory cells including a resistance variable thin film, e.g., a colossal magnetoresistive material. The thin film can be connected to an access device such as a diode, a field effect transistor (FET), or a bipolar junction transistor (BJT).
Memory cells can be programmed to a programmed level. The resistance of a PCRAM device can be altered by applying energy pulses to the phase change material, e.g., GST. For example, material properties of the GST may be altered by heating it with a programming current. Generally, a higher resistance level may be associated with a more amorphous state of the GST, and a lower resistance level may be associated with a more crystalline state of the GST. The resistance of a thin film RRAM device can be increased and/or decreased by applying positive and/or negative electrical pulses across the film.
To sense data stored on a resistance variable memory cell, the resistance of the memory cell may be sensed indirectly. In a binary system, a low resistance may correspond to a first value, e.g., 0, and a high resistance may correspond to a second value, e.g., 1. In some binary systems, a low resistance may correspond to a value of 1 while a high resistance corresponds to a value of 0. During a sensing operation, a sensed voltage and/or current corresponding to a resistive state of a selected memory cell may be compared to a reference voltage and/or current to determine if the cell has a greater or lesser resistive level thereby indicating the content of stored data. That is, the resistance of the selected memory cell may be sensed indirectly. For example, a transient response of a sensing circuit that is connected to the selected memory cell may be sensed, e.g., in response to a change in voltage or current.
A single level cell (SLC) can represent two programmed levels as represented by the binary digits 1 or 0. Memory cells can also store more than two binary digits, e.g., 1111, 0111, 0011, 1011, 1001, 0001, 0101, 1101, 1100, 0100, 0000, 1000, 1010, 0010, 0110, and 1110. Such cells may be referred to as multi state memory cells, multidigit cells, or multilevel cells (MLCs). MLCs can allow the manufacture of higher density memories without increasing the number of memory cells since each cell can represent more than one digit, e.g., more than one bit. Some non-volatile memories, such as flash, may achieve MLC functionality by storing one of a range of charges on a floating gate memory cell. Resistance variable memories may achieve MLC functionality by programming a memory cell to one of a range of resistances.